1. Field of the Invention
The present invention relates to an integrated control system for controlling steering and a power unit for use in a four-wheel steering vehicle in which the front wheels are steered while the rear wheels are turned.
2. Description of the Related Art
In the conventional four-wheel steering techniques, in a case where the rear wheels are turned in connection with the steering of the front wheels, the rear wheels are turned at an angle on the basis of the rear-wheel turning angle characteristics relative to the angle of the front wheels which are set beforehand and in accordance with the front-wheel steering angle. The four-wheel steering techniques which employ the rear-wheel turning angle characteristics that can be adequately corrected have been proposed in, for example, Japanese Patent Laid-Open NO. 61-220972. In this four-wheel steering technique, the rear-wheel turning angle characteristics are manually corrected in an in-phase direction when the speed of the vehicle is low so as to achieve improvement in the ease with which the vehicle can be turned. Correction of the rear-wheel turning angle characteristics in the in-phase direction means to correct the rear-wheel turning angle in a direction in which it increases when the speed of the vehicle remains the same.
The conventional four-wheel steering techniques in which the turning angle characteristics are automatically changed by the vehicle have been disclosed in, for example, U.S. Pat. No. 4,733,878. In this technique, the frictional coefficient of the surface of a road on which the vehicle is running is determined, and the turning angle characteristics are changed on the basis of the results of the determination.
In the four-wheel steering vehicles, the turning angle characteristics are corrected because they greatly affect the driving property of the vehicle.
Other factors which affect the driving property of the vehicle include the throttle opening characteristics. Recently, throttle valves of the type which are not mechanically connected to an accelerator pedal but are electrically driven by a pulse motor or the like have been widely used. Such electronic throttle valve control techniques are also disclosed in, for example, U.S. Pat. Nos. 4,508,078 and 4,729,356. U.S. Pat. No. 4,508,078 is directed to improvement of the response of an acceleration during acceleration of the vehicle. In this technique, a degree at which the accelerator pedal is depressed is detected. When this degree exceeds a predetermined value, the throttle valve is controlled such that a ratio of the changes in the throttle valve to the changes in the accelerator pedal increases. U.S. Pat. No. 4,508,078 is also directed to improvement of the response of a vehicle speed without deteriorating the acceleration, which is required when the vehicle is started or when the speed of the vehicle is decreasing. In order to achieve this, a desired opening of the throttle valve relative to the amount in which the accelerator pedal is operated is calculated in U.S. Pat. No. 4,508,078, and the throttle valve is controlled such that it is opened at the calculated desired opening. While the vehicle is being accelerated, the desired throttle valve opening is increased, and is then gradually decreased to the calculated desired opening within a predetermined period of time. Furthermore, the predetermined period of time is varied with the conditions under which the vehicle is driven for the purpose of improving the response of the vehicle speed.
Thus, it has been attempted to change the driving property of the four-wheel steering vehicle by correcting the turning angle characteristics in Japanese Patent Laid-Open No. 61-220972 and U.S. Pat. No. 4,733,878. In U.S. Pat. Nos. 4,508,078 and 4,729,356, changes of the driving property of the vehicle by means of the correction of the throttle opening characteristics relative to the opening of the accelerator pedal have been proposed. In other words, in a case where the rear-wheel turning angle characteristics are corrected in the four-wheel steering vehicle, correction of the rear-wheel turning angle characteristics and the throttle valve opening control relative to the amount in which the accelerator pedal is operated are performed independently and separately in the conventional four-wheel steering vehicle.